The present invention relates to a joint figuration of a hollow-shaped material to be used in friction stir welding. For example, the present invention relates to a hollow-shaped material made of aluminum alloy used in railway cars or structures and the like.
A friction stir welding method is a technique using a round shaft (called a rotary tool) being inserted into the welding region of members and moving the rotating rotary tool along the welding line, thereby heating, mobilizing, fluidizing and plasticising the welding region, and realizing a solid-phase welding of the members. The rotary tool comprises a small-diameter portion to be inserted to the welding region, and a large-diameter portion positioned at outer side thereof. The small-diameter portion and the large-diameter portion are coaxial. The boundary between the small-diameter portion and the large-diameter portion is slightly inserted into the welding region. The rotary tool is inclined rearwards against the progressing direction of the welding.
When the materials to be welded are hollow-shaped materials, the joint at the end portion thereof is constituted so that one end portion of one of the hollow-shaped material includes a rib for connecting the two face plates, and the face plates at the end portion of the other hollow-shaped material are superimposed on the above-mentioned rib. Also, centering on the above-mentioned rib, there are provided a convex portion projecting outwardly to each of the face plates. This region is friction stir welded.
This technique is disclosed in Japanese Patent National Publication of PCT Applications No. 09-508073 (EP 0752926B1), and Japanese Patent Laid-Open No. 09-309164 (EP 0797043A2).
When welding hollow-shaped materials together in order to obtain a structural body, the constitution of the joint is complex in figuration, unlike other members. Therefore, it is necessary to prevent the joint from decreasing in strength.
In order to prevent decrease in strength as a result of welding, a method of thickening the plate thickness of the joint compared to other members, regardless of the welding method, is known in the art. However, in order to reduce weight, the plate thicknesses of the general members areas thin as possible. Under such constitution, there exists large difference in rigidity between the general members constituting the structural body and the joint. The difference in rigidity between adjacent members results in occurrence of stress concentration, and occurrence of outward bending deformation by the bending moment. Therefore, a complicated stress field is formed, so that the strength of the region is decreased.
Also, when performing the friction stir welding method, the rotary tool is inserted into the hollow-shaped material, so that a large load acts on the direction of insertion. Such load is borne by ribs connecting the two face plates. Moreover, a force acts not only in the direction of insertion but also in the radius direction. Therefore, the above-mentioned rib is formed for the object of bearing the load generated in connection with insertion of the tool, and the plate thickness of the rib is increased. Moreover, the shape of the welding region becomes complicated. When the figuration of the joint becomes complex in order to increase strength, the difference in rigidity between the general members constituting the hollow-shaped material and the welding region becomes even larger. These result in increase in outward bending moment, and decrease in mechanical strength of the joint. As is seen from above, there exists inconsistency that when a joint is constituted so as to successfully obtain strength capable of bearing load generated during welding, the mechanical strength of the joint practically required is decreased owing to the difference in rigidity.
Moreover, when a tension or an outward bending deformation occurs to the hollow-shaped extrusion material of the structural body, such load is mainly borne by the face plate. That is, the cross-section at the intersection of the face plate and the above-mentioned rib is changed suddenly resulting in concentration of the stress, so that a high stress compared to other general members is generated. Therefore, a high stress compared to those generated in the general regions of the face plates is generated at the intersection of the face plate and the rib, causing decrease in strength.
Moreover, regardless of the welding method such as a friction stir welding or a MIG welding and the like, it is known in the art that the strength becomes the weakest at the heat-affected zone resulting from welding. Even in the case of friction stir welding having narrower heat-affected zone than MIG welding, a report is made in an outline of the lectures in national convention of the Welding Society (1998/10), in pages 332 through 333, that the width of the heat-affected zone is approximately 13 mm from the center of the tool. According to the present report, approximately the entire joint having complex figuration becomes the heat-affected zone.
As seen from above, the joint of the friction stir welding is remarkably decreased in strength, resulting from synergic effect of (1) bending moment inducted from the difference in rigidity (plate thickness), (2) stress concentration owing to complex figuration peculiar to the joint, (3) thermal effect as a result of welding, and (4) stress concentration resulting from connecting the rib with the face plate.
The present invention aims at providing a joint structure, which does not decrease in strength during friction stir welding.
The present invention relates to a hollow-shaped material, comprising:
two substantially parallel face plates;
a pair of first ribs for connecting said face plates together at each end portion of the face plates; and
a plurality of second ribs for connecting the face plates at positions closer to the opposite end portion than each of first ribs; wherein
the plate thickness of the face plates at connecting regions between the first ribs and the face plates are thicker than the plate thickness of connecting regions between the second ribs and the face plates; and
the thickened face plates are thickened by projecting toward the opposing face plate side.